From: Xuanqiang Luo rt_flush_dev() walks the per-CPU uncached route list and rewrites rt->dst.dev in-place to blackhole_netdev under spin_lock_bh(). This lock does not exclude RCU readers, which may load rt->dst.dev multiple times within a single rcu_read_lock() region. ip_rt_send_redirect() is a typical example: it reads rt->dst.dev three times to obtain in_dev, the L3 master ifindex, and net. A concurrent device unregistration can repoint rt->dst.dev to blackhole_netdev between those reads, making the reader combine state from two different net_devices — for instance, an in_dev from the real device but a netns and peer lookup from the blackhole device. ip_rt_get_source() has the same problem: it reads rt->dst.dev four times to obtain the output ifindex, the netns, and the source address, so a concurrent flush can cause the source selection to mix state from different devices. Take a single dst_dev_rcu() snapshot of rt->dst.dev at the start of each affected RCU reader and use that snapshot throughout, so concurrent flushes cannot cause mid-function inconsistency. Publish the in-place write in rt_flush_dev() with rcu_assign_pointer() to match the readers. Fixes: caacf05e5ad1a ("ipv4: Properly purge netdev references on uncached routes.") Signed-off-by: Xuanqiang Luo --- v2: - Use dst_dev_rcu() and dev_net_rcu() for the RCU readers. - Use rcu_assign_pointer() when publishing the uncached route device replacement. - Slightly adjust the commit message wording because this issue was found by inspection, not from an observed user-visible failure. v1: https://lore.kernel.org/all/20260630094250.29386-1-xuanqiang.luo@linux.dev/ net/ipv4/route.c | 29 +++++++++++++++++------------ 1 file changed, 17 insertions(+), 12 deletions(-) diff --git a/net/ipv4/route.c b/net/ipv4/route.c index 3f3de5164d6e5..57f38467e6d0c 100644 --- a/net/ipv4/route.c +++ b/net/ipv4/route.c @@ -873,6 +873,7 @@ static void ipv4_negative_advice(struct sock *sk, void ip_rt_send_redirect(struct sk_buff *skb) { struct rtable *rt = skb_rtable(skb); + struct net_device *dev; struct in_device *in_dev; struct inet_peer *peer; struct net *net; @@ -880,15 +881,16 @@ void ip_rt_send_redirect(struct sk_buff *skb) int vif; rcu_read_lock(); - in_dev = __in_dev_get_rcu(rt->dst.dev); + dev = dst_dev_rcu(&rt->dst); + in_dev = __in_dev_get_rcu(dev); if (!in_dev || !IN_DEV_TX_REDIRECTS(in_dev)) { rcu_read_unlock(); return; } log_martians = IN_DEV_LOG_MARTIANS(in_dev); - vif = l3mdev_master_ifindex_rcu(rt->dst.dev); + vif = l3mdev_master_ifindex_rcu(dev); - net = dev_net(rt->dst.dev); + net = dev_net_rcu(dev); peer = inet_getpeer_v4(net->ipv4.peers, ip_hdr(skb)->saddr, vif); if (!peer) { rcu_read_unlock(); @@ -1287,29 +1289,32 @@ void ip_rt_get_source(u8 *addr, struct sk_buff *skb, struct rtable *rt) { __be32 src; - if (rt_is_output_route(rt)) + rcu_read_lock(); + if (rt_is_output_route(rt)) { src = ip_hdr(skb)->saddr; - else { + } else { struct fib_result res; struct iphdr *iph = ip_hdr(skb); + struct net_device *dev = dst_dev_rcu(&rt->dst); + struct net *net = dev_net_rcu(dev); struct flowi4 fl4 = { .daddr = iph->daddr, .saddr = iph->saddr, .flowi4_dscp = ip4h_dscp(iph), - .flowi4_oif = rt->dst.dev->ifindex, + .flowi4_oif = dev->ifindex, .flowi4_iif = skb->dev->ifindex, .flowi4_mark = skb->mark, }; - rcu_read_lock(); - if (fib_lookup(dev_net(rt->dst.dev), &fl4, &res, 0) == 0) - src = fib_result_prefsrc(dev_net(rt->dst.dev), &res); + if (fib_lookup(net, &fl4, &res, 0) == 0) + src = fib_result_prefsrc(net, &res); else - src = inet_select_addr(rt->dst.dev, + src = inet_select_addr(dev, rt_nexthop(rt, iph->daddr), RT_SCOPE_UNIVERSE); - rcu_read_unlock(); } + rcu_read_unlock(); + memcpy(addr, &src, 4); } @@ -1565,7 +1570,7 @@ void rt_flush_dev(struct net_device *dev) list_for_each_entry_safe(rt, safe, &ul->head, dst.rt_uncached) { if (rt->dst.dev != dev) continue; - rt->dst.dev = blackhole_netdev; + rcu_assign_pointer(rt->dst.dev_rcu, blackhole_netdev); netdev_ref_replace(dev, blackhole_netdev, &rt->dst.dev_tracker, GFP_ATOMIC); list_del_init(&rt->dst.rt_uncached); -- 2.43.0